When you’re walking on the beach on a sunny day, polarized sunglasses come in handy: The glare off the water, even though it’s a reflection of unpolarized sunlight, is horizontally polarized, so it’s blocked by the vertically polarized lenses. The phenomenon is the result of how light reflects off a dielectric, so glare off metal surfaces is different. If you had full polarization vision, you could readily tell whether a shiny object in the distance is a pool of water or a sheet of steel.
What would “full polarization vision” entail? To completely describe light’s polarization state—whether linear, circular, elliptical, or partially or completely unpolarized—it takes four numbers, often represented as the four-component Stokes vector, named after George Gabriel Stokes, who worked out the idea in the 1850s. To get a fully polarized view of a scene, therefore, you’d need in effect to image it four times.
But that fourfold...
